\begin{spreadtab}{{tabular}{r*6c}} % or \spreadtab{{tabular}{r*6c}}
	@Ages    & 10                                                      & 11 & 12 & 13 & 14 & 15 \\
	@Number  & 5                                                       & 8  & 20 & 55 & 9  & 3  \\\hline
	@Average & \multicolumn{6}{l}{:={sumprod(b1:g1;b2:g2)/sum(b2:g2)}}
\end{spreadtab} % or \endspreadtab
